The invention relates to a method for manufacturing uncoated industrial woven fabrics with specifically adjusted air permeability from synthetic filament yarns.
Uncoated industrial woven fabrics with specifically adjusted air permeability are highly important in some application fields. Low air permeability is required for sail and parachute fabrics and for fabrics used for the contact portion of air bags.
Air bags are frequently made from two fabric portions having differing air permeability. For the contact portion of the air bar, which faces the vehicle occupant when the air bag is released and against which gas entering the air bag flows, a very low air permeability of less than 10 l/dm.sup.2.min (measured at 500 Pa test differential pressure) is required. To allow soft cushioning of the vehicle occupant during a vehicle collision, however, a portion of the gas which inflates the air bag must be able to escape when the air bag contacts the vehicle occupant. For this reason, the air bag has a second fabric portion with higher air permeability than the contact portion of the air bag. The second fabric portion is normally designated as the filter portion. In addition to allowing gas to escape from the air bag, this fabric portion also has the task of intercepting particles in the gas used to inflate the air bag, which to some extent are very hot. The air permeability values for the filter portion of the air bag are typically between 20 and 120 l/dm.sup.2.min (measured at 500 Pa test differential pressure), depending on the type of vehicle. The manufacturing conditions for fabrics requiring both high and low air permeability must be precisely matched to the required air permeability.
Uncoated industrial woven fabrics with specifically adjusted air permeability are of great importance for air bags, gas filtration, manufacturing jump cushions, and other applications.
An air bag may be woven in one piece, with regions of differing air permeability created by varying the fabric density on a power loom, or two different fabrics having differing air permeability values may be sewn together to create the air bag.
Coated woven fabrics are employed for the contact portion of air bags and for other applications of industrial woven fabrics where low air permeability is required. In addition to the high manufacturing costs, these fabrics have additional disadvantages, and this is particularly true for the air bag. For this reason, methods have been developed for manufacturing uncoated woven fabrics with either low or specifically adjusted air permeability. Easily conductible wet processes, with appropriate drying conditions are described in EP-A 436 950 and EP-A 523 546. By means of specifically initiating shrinkage in these cases, fabric compaction is achieved in an easily influenced and controllable manner. Another approach to manufacturing uncoated industrial woven fabrics with low air permeability uses calendaring methods as described in EP-A 314 867 and EP-A 453 678.
Despite the relatively low manufacturing costs for uncoated industrial woven fabrics having low or specifically adjusted air permeability, there is still a need to reduce the manufacturing costs of these fabrics even further and to develop an even more cost effective method.
Surprisingly, it has now been found that it is possible to integrate the wet process for shrinkage initiation into the weaving process. Previously, the wet process for shrinkage initiation was conducted discontinuously with the weaving process. The continuous process of weaving and shrinkage initiation allows a further significant reduction in the costs of manufacturing uncoated industrial woven fabrics with specifically adjusted air permeability values.
Methods for incorporating a wet process into the weaving process have been disclosed. In DE-A 2,849,596, a "shrinkage bath" is mentioned, although the meaning of this term is not explained. As indicated in FIG. 6 and FIG. 7 of the cited patent publication, this bath is situated downstream from a drying zone. The bath is obviously intended for applying a finish to the fabric. Due to the wet zone's location downstream from the dryer, this method is not suitable for manufacturing industrial woven fabrics with specifically adjusted air permeability. Moreover, this document provides no information concerning the process parameters to be used for industrial woven fabrics of this type.
A similar process is described in AT-B 240 807. In this reference, according to FIG. 3, the industrial fabric is passed through a liquid bath after leaving the weaving zone, and a liquid metal bath is preferred. In this method, shrinkage initiation is not evident from either the description or the drawings. A drying apparatus is also not provided. This document contains no information about parameters for manufacturing industrial woven fabric with specifically adjusted air permeability.